The present invention relates generally to Sagnac rotation-sensing ring interferometers and more particularly to a Sagnac rotation-sensing ring interferometer which operates at its maximum sensitivity point for small rates of rotation (.OMEGA.).
The Sagnac ring interferometer, which employs a long single-mode fiber-optic loop, has shown promise as a passive gyroscope for navigational purposes. The properties of Sagnac fiber-optic gyroscopes are described in an article by Rashleigh and Burns, entitled "Dual-Input Fiber-Optic Gyroscope", Optics Letters, Vol. 5, No. 11, p. 482, Nov. 1980. Typical Sagnac fiber-optic gyroscopes include means for introducing light beams into both ends of the fiber-optic loop and means for measuring the relative phase shift of the light beams exiting the fiber-optic loop in order to determine the rate of rotation (.OMEGA.) in the plane of the fiber-optic loop. An optical coupler splits an incoming light beam into two counter-propagating beams in the fiber-optic loop and provides for interference between the light beams exiting the fiber-optic loop.
The interferometer operates at maximum sensitivity, or quadrature, for small rates of rotation when the output beams of the optical coupler have equal intensity at zero rotation rate. Prior art couplers having two input terminals and two output terminals, (2.times.2), do not operate at quadrature unless a non-reciprocal .pi./2 phase shift is induced into the optical path of the beams. The means needed to induce the non-reciprocal .pi./2 phase shift increase the complexity and susceptibility to noise of prior art interferometers using (2.times.2) couplers.
A (3.times.3) coupler as disclosed, for example, in U.S. patent application Ser. No. 320,999 by S. K. Sheem operates at quadrature for small rates of rotation, but the increased separation of the optical paths providing interference between the light beams exiting the fiber-optic loop decreases the coupling between the beams and requires a longer coupling length. Attempts to arrange the optical paths in non-planar configurations to increase coupling destroys the symmetry required for fabrication by integrated optics techniques.